Data networks provide for the communication between a number of users due to the networking, that is to say the connection of the individual users to one another. Communication here means the transmission of data between the users. The data to be transmitted are sent out as data telegrams, i.e. the data are packed together to form a number of packets and are sent via the data network to the corresponding receiver in this form. They are, therefore, also called data packets. The term transmission of data is used completely synonymously to the above-mentioned transmission of data telegrams or data packets in this document. The question of networking itself is solved, for example in the case of switched high-performance data networks, especially the Ethernet, by in each case at least one switching unit being connected between two users which is connected to both users. Each switching unit can be connected to more than two users. Each user is connected to at least one switching unit but not directly to another user. Users are, for example, computers, stored program systems (SPS) or other machines which exchange, especially process, electronic data with other machines. In contrast to bus systems in which each user can reach any other user of the data network directly via the databus, the switched data networks are exclusively point-to-point connections, i.e. a user can only reach all other users of the switched data network indirectly by corresponding forwarding of the data to be transmitted by means of one or more switching units.
In distributed automation systems, for example in the field of drive engineering, certain data must arrive at certain times at the users intended for them and must be processed by the receivers. These are called real-time-critical data or data traffic since any untimely arrival of the data at the destination leads to unwanted results at the user. According to IEC 61491, EN61491 SERCOS interface-brief technical description, a successful real-time-critical data traffic of the abovementioned type can be guaranteed in distributed automation systems.